1) They don't puke acid fumes all over and corrode everything nearby, including themselves.

2) They don't self discharge at about 10%/month.

3) They have a very high charge acceptance rate, if your charging system is up to it.

They are however expensive and need a well regulated charger. They also seem to need to be fully charged periodically (like once or twice a month) or they sulfate prematurely. Many owners never fully charge their batteries - it requires a very long engine run or some other source of charge. Flooded cells will survive those two abuses better. For amp hours delivered over their life, flooded cells are hard to beat. However if you have a proper charging system, AGMs will last as long or longer, require less maintenance (like, none), cause no acid damage to anything, and recharge faster.